A) the focal length of the objective has to be increased
B) both the focal length and aperture of the objective has to be increased
C) the wavelength of light has to be decreased
D) the aperture of the objective has to be increased
Correct Answer: B
Solution :
\[{{f}_{1}}=\]frequency of first fork. \[{{f}_{41}}=\]frequency of 41st fork. \[{{f}_{41}}=2{{f}_{1}}\] Between 1st and 41st there are 40 intervals each interval produces 5 beats. \[{{f}_{41}}-{{f}_{1}}=40\times 5\] \[2{{f}_{1}}-{{f}_{1}}=200\] \[{{f}_{1}}=200\,Hz\]
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